A low protein feed, fed only once, or twice a day. Lots of roaming space also.Do you what she she should be feeding twice daily? I am pretty sure that holding off treats and fruit is important to. Only her feed that she gets twice daily, she needs to loose weight.
I've seen this method, before, but never had Cornish X to try it on. I think someone who breeds them may know more then you, & I though.